Caldo ceramic filters are high temperature devices for the continuous removal of particulates from air to other gases. Caldo supplies a range of filter sizes and for larger gas flows employs a modular approach, combining several vessels of proven design to achieve the required capacity. The addition of sorbent powder enables the filters to function as dry scrubbers in waste gas applications.

The ceramic filter element is vacuum formed from alumino-silicate ceramic fibers as a 1000-mm long tube, flanged at one end and closed at the other. Its composition is stable under extreme conditions of temperature and/or chemical corrosion. The element can withstand a temperature of 900° C, but if the housing is fabricated in mild steel the filter operating temperature is limited to 450° C. Stainless steel construction allows higher temperatures, albeit at a cost. The elements can resist all chemical attack with the exception of HF.

The elements hang vertically in the filter vessel from the header plate, which separates the clean and dirty compartments. The hot gas is sucked through the filter medium from outside to inside, depositing the particles on the outer surface of the medium. At controllable intervals a sharp pulse of air is blown back down the inside of the filter element causing a momentary reversal of flow. This reversal causes the accumulated solids to be detached from the outer surface of the filter elements. The solids fall into the hopper section of the vessel from where they may be discharged.

The capacity of the filter is determined by the number of filter elements contained in the filter vessel. Caldo has proven standard designs for 64, 120, 214 and 256 element filters. If the 256-element design is not large enough, two or more vessels may be installed in parallel.

Acid gases can be very effectively removed by the addition of lime or sodium bicarbonate to the gas upstream of the ceramic filter. Both lime and sodium bicarbonate react well with HCl, HF and HS₂O₄ and are claimed to remove a proportion of any NO_x that is present.

Ceramic filters are useful on nearly all high temperature processes where the hot gas may contain smoke, fume or dust. Typical applications include: incineration of all types of waste, cleaning of fuel gases from pyrolysis or gasification processes, protection of catalysts in chemical processes, gases from metallurgical furnaces, and product collection.